Several techniques are available for creating images of the retina. One of conventional methods is identified as a fundus photography, where a full-field image of the retina is obtained by flood illumination. Another well-known method for such image generation is a scanning laser ophthalmoscopy (SLO) procedure. For example, SLO utilizes horizontal and vertical scanning mirrors to scan a specific region of the retina and create raster images. SLO imaging relies on the principle of confocal microscopy. The light is focused on the retina, and the reflected light is imaged back onto a pinhole to reject the out of focus light reflected from the eye. Due to their confocal nature, images generated by SLO show a higher contrast than fundus photos. Another difference is that eye movements during acquisition of a single frame likely results in geometric distortions of the resulting image. In fundus photographs, motion during illumination results in a blurred image.
For example, images generated by SLO have been used to monitor eye motion, and provide feedback to correct for eye motion in, e.g., optical coherence tomography (OCT) imaging. In such exemplary OCT setup, a newly acquired frame is compared to a reference frame and the displacement that minimizes the difference between these two frames is assumed to optimally correct the motion of the eye. However, as indicated above, eye movements during the acquisition can produce geometric distortions of the acquired image. As the acquisition of a single frame requires, e.g., approximately 30 milliseconds, these distortions may frequently occur. Such distortions, therefore, have a negative impact on the tracking ability of the system, especially if these eye movements occur during the acquisition of the reference frame. Since subsequent frames are generally compared to the reference frame to determine the eye motion, the distortions in the current frame or the reference frame can lead to the incorrect motion determination and the incorrect motion correction signals to the secondary imaging modality.
The tracking speed of SLO-based eye tracking is generally limited by its frame rate. However, sub-frames may be used instead of full-frames, thereby likely increasing the tracking speed. Due to the employed scanning mechanism of SLO, these sub-frames are obtained by either horizontally or vertically dividing the full frame into sub-frames. Such rectangular sub-frames can cover different parts of the retina. This can result in a different performance in horizontal and vertical directions (depending on the orientation of the rectangular sub-frame). Further, the performance would then depend on which sub-frame is being analyzed, because some parts of the retina contain more cues for the displacement estimation than other parts.
Accordingly, there may be a need to address at least some of the above-described deficiencies.